A 250‐year cycle in naked‐eye observations of sunspots

Abstract

The state of the sun strongly affects terrestrial phenomena. While the eleven‐year solar cycle was recognized in the XIX century, the lack of direct data concerning our star in historical times has made it difficult to determine the characteristics of solar variability on a secular scale. We here construct a series of yearly sunspot numbers as observed by eye in remote times and recorded in historical documents covering the period 165 BC–1918 AD. Using different spectral analysis methods, we find a 250‐year cycle which appears to have a solar origin.